In the development of gas turbine engines, it is beneficial to measure the vibration (both amplitude and frequency) of the rotating blades. From such measurements, the induced stresses in the blades may be determined and action taken to avoid stresses which are high enough to cause damage to the blades.
It is known to mount strain gauges on rotating turbine blades to provide information about the amplitudes and frequencies of vibration of the blades. Strain gauges may be provided on each blade, and connected to a telemetry system mounted on the rotor, which transmits the measurements from the rotor. Due to the number of strain gauges required to fully determine the vibration, the telemetry system can be complex, large and time consuming to install within the rotor. Also due to this complexity, it is common practice to instrument a sampling of blades instead of all blades, and therefor blade to blade variation is often not detected An alternative technique for characterizing blade rotation is “tip timing” in which a non-contact probe mounted on the turbine casing is used to measure the time at which a blade passes This time is compared with the time at which the blade would have passed the probe if it had been undergoing no vibration. This is termed the “expected arrival time” and can be calculated from the rotational position of the particular blade and the velocity of the rotor.
The difference between the expected arrival time and the actual arrival time can be multiplied by the turbine blade tip velocity to give the displacement of the blade from its expected position. Data from a plurality of sensors can be processed to obtain the amplitudes and frequencies of vibration of the blades
The relationship between blade deflection and stress can vary dramatically over a small area of the blade. Accordingly, there is currently an unmet need for a tip timing system that provides information related to a tip timing sensor's location over a rotating blade.